What is RGB, what is CMYK? And how to convert RGB to CMYK?

You will probably already know that RGB and CMYK stand for two different ways of representing colors or creating colors.

RGB is the abbreviation for Red, Green, Blue. All RGB color spaces are based on these three primary colors.

CMYK stands for Cyan, Magenta, Yellow, Key. The CMYK color model thus results from three basic colors plus black as the fourth color, traditionally referred to as key.

In the next section, we must first address the basics. Some knowledge of color theory is simply indispensable for understanding the subject.

Basics of color theory

Additive color mixing

RGB colors are light colors and function additively, i.e. by adding color components. From the phenomenon of the rainbow, we know that light can be broken down into spectral colors. The primary colors correspond to three different wavelength ranges. We refer to short-wave light as blue, medium-wave light as green, and long-wave light as red. The superposition of red, green and blue light in different proportions is therefore the origin of comprehensive color diversity.

Each pixel of this PNG file, which is only 6 kB in size, has a different color. With a side length of 1000 pixels, this results in a million different colors.

Accordingly, in the human eye there are three different color receptors, so-called cone cells, each of which is specialized in one wavelength range.
Let’s look at a few additive color mixtures. For example, red and green become yellow, red and blue become magenta, and the superposition of all three primary colors produces white.

additive color mixing

Subtractive color mixing

CMYK inks are printing inks and therefore solid colors. They do not illuminate themselves but require a light source to make them visible. Body colors work subtractively. This means that certain parts of the color spectrum are removed to produce the desired color. If you relate body colors to light colors, cyan acts as a subtrahend of long wavelengths, in other words as a red filter. Magenta acts accordingly as a green filter, and yellow as a blue filter.

subtractive color mixing (without key)

In the subtractive model, the color white is the color from which nothing has been subtracted (subtracted). In the CMYK color model, this means that the values for cyan, magenta, yellow and black are each set to 0.

Screenshot from Adobe Photoshop. White displayed with RGB and CMYK values.

What distinguishes screen colors from printing colors?

We already know that colors on screens are light colors, while in printing we are dealing with body colors. But what does that mean in detail?

For this purpose, let’s look at how a screen pixel is constructed. It consists of a red, a green and a blue light unit.

LC display under the microscope: The RGB light units of individual pixels become visible.

The light intensity can now be set for each unit. Viewed with some distance, this results in the specific color of the pixel.
(Learn more about pixels and screen resolution here).

When digitally printing photos or documents in color, we are dealing with inks that, in the vast majority of cases, are applied to a white sheet of paper; more precisely, to a sheet that appears white to us in the light. This means that initially all wavelength ranges are reflected diffusely*. During the printing process, ink or toner powder is then applied to the white and slightly roughened paper surface in glaze layers, first cyan, magenta and yellow, and finally black.
* A mirror also does this, but directly, because it has a particularly smooth surface.

Demonstration of the four printing inks. Bottom left: Overlay of CMY, bottom right: CMY + K

Convert RGB to CMYK

Basically, you only need to delve deeper into color spaces, color models, and color profiles if you’re professionally involved in printing photos and graphics; after all, the RGB color values are converted one way or another when a print job is created – and minor color deviations are not a problem for most users.

Keep deviations low

Graphic designers, professional photographers, advertising and printing technicians, etc. naturally want to leave nothing to chance at this point and keep deviations to a minimum. I deliberately say “keep them low,” because they can’t be completely eradicated. Even with ideal calibration of the monitor, perfect shielding of ambient light and use of suitable color profiles, a printed image will always look different from the one on the screen, simply because it does not glow but reflects incident light. In addition, some color information is always lost when converting RGB to CMYK color values, since the RGB spectrum is broader and cannot be represented one-to-one in the CMYK color model.

Gamut: LAB / RGB / CMYK

The graphic shows the different tonal range (gamut) of RGB and CMYK against the background of the LAB color space, which includes all colors that can be perceived by the human eye.

Devil in the details

The color deviations that occur when converting from RGB to CMYK are minimal, probably barely visible to most, and not a dramatic matter. But when you think of global brands, it’s hardly surprising that no deviations are tolerated there. At Coca-Cola, for example, they go so far as to use five colors for official print products. CMYK + Cola Red – because the very specific red of the brand cannot be produced from the glaze of CMYK printing inks, but must be mixed separately.

Convert CMYK to RGB

Conversely, CMYK colors can of course also be converted to RGB. Here, too, the conversion is only approximate due to the different pitch range. This is just as negligible for most users, especially when you consider the deviations that different monitor settings alone can lead to.
Tip: On the following website you will find a small tool that allows you to easily convert individual colors or color codes in both directions (RGB to CMYK and CMYK to RGB): rapidtables.com

Different types of RGB

sRGB

sRGB stands for Standard RGB. The specifications of the color space were defined in 1996 as a result of a cooperation between Hewlett-Packard and Microsoft. sRGB was able to establish itself as an industry-wide standard as planned. However, it has minor weaknesses in terms of range. Thus, some colors possible in CMYK printing cannot be represented in sRGB.

Adobe RGB

Adobe Systems followed suit and developed a color space with a somewhat expanded gamut in the green and blue-green color tones. The aim was to further reduce color deviations between screen output and print. In 1998, the specifications were published. Since then, Adobe RGB is often preferred over sRGB by image editors and professional designers.

CMYK color profiles

What is true for RGB is also true for CMYK. Here, too, there are different profiles depending on whether a file is prepared for offset printing (long runs) or as a digital print (dry toner / ink). The CMYK profile most commonly used in Europe is called ISO Coated v2. It was developed for industrial offset printing on matte or glossy paper.
For digital printing, the eciCMYK v2 standard is more suitable. However, we do not want to go into this in any further detail here. At this point, it should suffice to point out that the printing industry has a large number of CMYK color profiles available, each with its own device- or material-specific orientation.

What is the difference between color model, color space and color profile?

A whole book could probably be written about this question. Here are the short answers:

Color model

A color model is a mathematical method for describing colors independent of physical devices or output media.

Color space

In a color space, real displayable colors are mapped to the values of a color model. A color space thus describes all the colors that can be reproduced in a specific medium (screen, print, projection, etc.). So sRGB and Adobe RGB are color spaces that use RGB as a model.

Color profile

Color profiles help to convert color values. They are device-specific and serve the purpose of maintaining the color impression across different output media. Standardized color profiles can be recognized by the abbreviation ICC. This stands for International Color Consortium.

Color management with the Media Hub from teamnext

If hundreds of images have to be delivered every day, it would of course be tedious to apply the appropriate color profile to each file individually. With a professional digital asset management system such as teamnext | Media Hub, it is therefore possible to specify the color space for all images uniformly during file export. Here you have the choice between sRGB, CMYK and YcbCr. The latter is a color space that is used i. a. for the encoding of JPEG files.

But finally the proof is in the pudding

If we have made you curious and you just want to try out the various functions of our image management, you can get started immediately with a free 14-day trial period for teamnext | Media Hub. In addition, you can book an appointment for a free online product demo with one of our experts at any time. Simply use our contact form for this purpose.